feat(10-02): add GraphDirectoryUser model and IGraphUserDirectoryService interface

- GraphDirectoryUser positional record with DisplayName, UPN, Mail, Department, JobTitle
- IGraphUserDirectoryService.GetUsersAsync with clientId, IProgress<int>?, CancellationToken
- Follows existing GraphUserSearchService namespace pattern

SharepointToolbox/Core/Models/GraphDirectoryUser.cs

@@ -0,0 +1,12 @@
+namespace SharepointToolbox.Core.Models;
+
+/// <summary>
+/// Represents a directory user returned by <see cref="SharepointToolbox.Services.IGraphUserDirectoryService"/>.
+/// Used by Phase 13's User Directory ViewModel to display and filter tenant members.
+/// </summary>
+public record GraphDirectoryUser(
+    string DisplayName,
+    string UserPrincipalName,
+    string? Mail,
+    string? Department,
+    string? JobTitle);